The present invention relates to an injection molding method and apparatus and, more particularly, to an injection molding method and apparatus which are suitably used for, for example, forming a lining inside the measuring pipe of an electromagnetic flowmeter and molding a resin pipe.
In an electromagnetic flowmeter for measuring the flow rate of a conductive fluid flowing in a measuring pipe by using the electromagnetic induction phenomenon, to prevent a shortcircuit between an electromotive force produced in the fluid and the measuring pipe made of a nonmagnetic material such as stainless steel, the inner surface of the measuring pipe, which is the liquid contact surface, and the surfaces of the flanges, integrally formed on the two end portions of the measuring pipe, to which pipes are connected (to be referred to pipe connection end faces hereinafter) are generally covered with a lining material. As a lining material, an insulating material such as a fluoroplastic material is generally used because heat resistance, corrosion resistance, electric insulating properties, and the like are required. Such a lining material is formed on the inner surface of the measuring pipe and the pipe connection end faces of the flanges by injection molding.
When a measuring pipe covered with such a lining is to be molded by a transfer molding machine, the measuring pipe body is loaded in a mold, and the mold is heated to the melt temperature of a lining material or higher. Thereafter, the molten lining material is pressurized and injected into the mold to cover the inner surface of the measuring pipe body and the pipe connection end faces of flanges with the lining material.
In lining a measuring pipe, a fluoroplastic material used as a lining material has poor adhesion with a metal, and hence tends to peel off from the measuring pipe. For this reason, as disclosed in Japanese Patent Laid-Open Nos. 61-294316 and 61-271419 and Japanese Utility Model Publication No. 2-28411, a reinforcing pipe formed by a perforated plate generally called a punching plate is mounted inside the measuring pipe body in advance, and the mounted reinforcing pipe is covered with a lining material. This increases the mechanical bond strength between the lining material and the measuring pipe to prevent the lining material from peeling off, and also prevents deformation and the like of the lining material due to changes in temperature and pressure inside the measuring pipe.
FIG. 16 shows a conventional injection molding apparatus used to mold a measuring pipe. Referring to FIG. 16, an injection molding apparatus 1 includes a mold 4 constituted by a upper mold 2 and lower mold 3 and an injection nozzle (not shown). The lower mold 3 is made up of a first lower mold 5, second lower mold 6, and core 7. A molten molding material 9 such as a fluoroplastic material is pressurized and injected into a cavity 8 formed in the mold 4 to line the inner surface of the second lower mold 6 and an annular recess portion 15 formed in a pipe connection end face 10a of a flange 10, thereby extracting the second lower mold 6 as a measuring pipe 11. That is, the injection molding apparatus 1 uses the measuring pipe body as the second lower mold 6, and lines the measuring pipe body by outsert molding. Note that reference numeral 16 denotes a reinforcing pipe mounted inside the second lower mold 6 through a spacer 17; and 18, a cooling circuit (cooling medium route) of the mold 4.
In injection molding of a measuring pipe, if the clamping pressure (closing force) of the mold 4 is insufficient with respect to a molding pressure (injection pressure), the molten molding material 9 leaks from a joint portion 12A between the upper mold 2 an the second lower mold 6 and a joint portion (to be referred to as a seal portion hereinafter) 12B between the first lower mold 5 and the second lower mold 6. In this case, the leaking portion of the molding material 9 hardens and remains as a burr on the surface of the molded product. If the molding material 9 excessively leaks, a sufficient amount of resin is not charged into the mold 4, resulting in incapability of molding. In order to prevent this, for example, the mold 4 is closed with a plurality of bolts 13 and a closing plate 14 or by using a hydraulic closing mechanism to prevent the seal portions 12A and 12B from opening.
Letting D (cm2) be the total projection area of a molded product, P (Kg/cm2) be the injection pressure, and W (Kg) be the closing force, the following condition must be satisfied to prevent the seal portions 12A and 12B from opening:
DP less than W 
Note that the total projection area D indicates the area of the inner wall surface of the mold 4 which is in contact with the molten resin in the direction of closing (the axial direction of the bolt 13). Therefore, xe2x80x9cDPxe2x80x9d indicates a component force, in the direction of closing, of the force which the mold 4 receives from the molten resin.
Such an injection molding method is disclosed in xe2x80x9cTRANSFER MOLDING METHOD FOR THERMOPLASTIC RESIN AND METHOD FOR COATING BODY WITH THERMOPLASTIC RESINxe2x80x9d, Japanese Patent Laid-Open No. 5-147061. According to this reference, the statement xe2x80x9cthe extrusion molding machine is coupled to another pot to which a target body-mounted mold is coupled/closedxe2x80x9d indicates that this technique includes some closing means although it is not described in detail.
A mold and molding apparatus and method which are used to transfer-mold a fluoroplastic lining on the inside of a pipe are disclosed in detail in xe2x80x9cPreliminary Information from Plastic Technical Services Laboratory about DU PONT TEFLON FLUOROCARBON RESIN, DU PONT TEFZEL FLUOROPOLYMER FLUOROCARBONS DIVISION, PLASTICS DEPARTMENT, E. I. DU PONT DE NEMOURS and CO. (INC.), WILM., Del. 19898 PIB #36 (Revised), August 1973xe2x80x9d. According, page 23, lines 10-13, xe2x80x9cIt is not necessary to bolt the melt pot assembly to the mold as the melt pot area is much greater than the gate area. As a result, the xe2x80x9csealing forcexe2x80x9d between the mold and nozzle is far greater than the force exerted by the melt at the nozzle/sprue bushing interface.xe2x80x9d, the sealing property of the interface between the nozzle of the melt pot and the sprue hole of the mold can be ensured by using the injection pressure exerted on the molten material. However, the mold itself must be closed by bolt fastening as in the prior art shown in FIG. 16.
As described above, in the conventional injection molding apparatus 1, the mold 4 is closed by the bolts 13 and closing plate 14 or a hydraulic closing mechanism to prevent the molding material 9 from leaking out from the seal portions 12A and 12B. In the closing operation using the bolts 13 and closing plate 14, however, the operator must mount/dismount the closing plate 14 for every molding operation, resulting in a heavy load on the operator and significant deteriorations in operability and productivity.
The use of a hydraulic closing mechanism will increase the size of an injection molding apparatus itself, resulting in an increase in cost.
It is an object of the present invention to provide an injection molding method and apparatus which do not require a means dedicated to mold closing, e.g., bolts or a hydraulic closing mechanism.
It is another object of the present invention to provide an injection molding method and apparatus which prevent plastic deformation and destruction of a mold.
In order to achieve the above objects, according to the present invention, there is provided an injection molding apparatus comprising a vessel which stores a molding material, the vessel having a nozzle hole which is formed in a bottom portion to inject the molding material and a first seal surface formed around the nozzle hole, a mold which is made up of a plurality of stacked members stacked on a lower side of the vessel and is filled with the molding material injected from the vessel, the mold having a sprue communicating with the nozzle hole and a second seal surface formed around the sprue, and the stacked members respectively having third seal surfaces at portions where the members are in contact with each other, a pusher for pressurizing the molding material in the vessel, and thrust providing means for providing a thrust to the pusher to pressurize one of the pusher and the mold through the molding material in the vessel, the thrust providing means bringing the first, second, and third seal surfaces into tight contact with each other by providing a downward thrust to the pusher with respect to the mold.